Fragile X Syndrome (FXS) is the most frequent form of inherited mental retardation, and characterized by an abundance of immature postsynaptic dendritic spines in adult cortical neurons. The objective of this project is to examine spine dynamics and morphology in different cortical regions and layers of the brain during disease progression in a mouse model of FXS (Fmr1 KO), and to explore potential therapeutic strategies targeting different signaling pathways and cell types to correct both synaptic structural and learning behavioral defects. Using transcranial two-photon microscopy, in combination with molecular approaches to manipulate gene expression in individual cortical neurons in vivo, we propose 3 aims. Aim 1 systematically examines altered dendritic spine morphology and dynamics in the cortex of developing and adult Fmr1 KO mice. It will directly test the current hypothesis that FXS results from a developmental defect in spine pruning and maturation. Aim 2 dissects and compares the cellular mechanisms of two potential therapeutic strategies for FXS. Aim 3 investigates neuronal and glial roles in abnormal development of the dendritic spine of cortical neurons in Fmr1 KOs. Results from the proposed studies will provide much needed details about spine dynamism during the pathogenesis of FXS in mice. Such information will help to elucidate the cellular mechanisms for this disease and potentially lead to identification of new cellular targets for treatment.